A power amplifier (PA) is used in a wireless communication device such as a cellular radiotelephone to amplify radio frequency (RF) signals so that the device can communicate with a fixed site transceiver. Considerable power in a wireless communication device is dissipated in the power amplifier. For example, in a portable cellular radiotelephone, a significant percent of the power dissipation is in the power amplifier. Efficiencies of a power amplifier significantly depend upon variations in output power. One problem associated with designing a high-efficiency power amplifier is adequately compensating for these output power variations. Improving the power amplifier efficiency is essential to increasing the operating time for a given battery of the wireless communication device.
Wireless communication devices typically transmit radio frequency signals at a plurality of power levels. For example, some cellular radiotelephones require seven 4 dB steps in output power of the radio transmitter. However, newer cellular telephone systems have required additional power levels. The efficiency of the power amplifier significantly varies over the output power range. Because current drain efficiency of the power amplifier is most affected at a higher output power, the power amplifier is typically designed to maximize efficiency at higher output power levels. One technique to improve power efficiency requires switching the quiescent current of the power amplifier in response to a power amplifier output step change. At the lowest power step, the power amplifier is normally in class A mode of operation. By changing the bias conditions of the power amplifier at the lower steps, the power amplifier could be kept in class A/B mode with a corresponding improvement in efficiency.
In a conventional two-stage amplifier, a variable voltage supply on the first stage drain controls the output power of the amplifier via adjusting the gain of the first stage, commonly called single drain control. The second stage is biased class A/B and has constant Vds and Vgs. This control configuration provides optimum efficiency for high output power levels only. At lower output power levels, the amplifier current drain is high due to the total current drain asymptotically approaching the second stage quiescent current. In addition, the dynamic range in this configuration is limited to 25 dB, inadequate for achieving certain power levels required in a system having a wider dynamic range, such as power levels 8, 9, and 10 as set forth in the IS-91 specification published by the Electronics Industry Association/Telecommunications Industry Association located at 2001 Pennsylvania Ave., N.W. Washington, D.C. 20006.
Other conventional devices using dual gate control use fixed supply voltages at the drains of both stages. Varying Vgs of both stages controls output power by changing the gain of each stage. While dual gate control may provide a wider dynamic range, the MESFETs are typically operated in the pinchoff region to provide the dynamic range, compromising stability of the amplifier at low power levels.
Accordingly, there is a need for a method and apparatus for amplifying a radio frequency signal with greater efficiency over a wide dynamic range.